1. Field of the Invention
The invention relates to a method and apparatus for drilling a tunnel, wherein there are two independent forces; the one driving the protecting tube and the other driving the drilling apparatus by means of a rotating spiral tube.
2. Description of the Prior Art
A previously known drilling apparatus as described in applicant's former patent application No. FI-891706, is one in which the protecting tube is forced into the excavated drilling head advances through the tunnel portion as the tunnel. Also the force for the tool in the drilling head is transmitted over the protecting tube. The conveying tube rests against the inner surface of the protecting tube and moves forward along with the protecting tube. There is a thrust bearing in the drill head, and so the force over the protecting tube is transmitted entirely through the thrust bearing as a force for the tool forward drive.
U.S. Pat. No. 2,669,441 the tools and the thrust bearing are in the working pit. No protecting tube is driven into the tunnel but the force into the drill head is simply brought forward by a rotating conveyor pipe.
The disadvantage of the prior art devices is lack of control of the force. On driving the drill head forward with the protecting tube, the required force changes as the length of the protecting tube grows and because of friction from different soil types. This means that the farther the drilling advances the more the information about the impact of tool forces against he front wall of the tunnel diminishes and possible obstacles cannot then be detected. Therefore, the risk of tool damage is great. In U.S. Pat. No. 2,669,441 drilling is possible only in rock or soil that needs no protecting tube to support the tunnel.